Agar, a 19-year-old Western Australia left-arm spinner, was only brought to England originally as a "development player" for the first two Ashes Tests.
But in a dramatic move involving Australia chairman of selectors John Inverarity, on tour selector and wicketkeeping great Rodney Marsh and Lehmann himself, Agar was preferred Wednesday instead of off-spinner Nathan Lyon, a veteran of 22 Tests.
The stunning selection did not leak from the tourists' camp until fast bowling great Glenn McGrath was seen handing Agar his 'Baggy Green' cap, in a traditional presentation ceremony for Australia players making a Test debut, shortly before Wednesday's toss.
"For a 19-year-old kid, it's great," added Lehmann, a former Australia batsman.
"He's excited and you saw him when Glenn McGrath presented him with his cap, he had a tear in his eye and it's a big moment for him.